The present invention relates to heat shrinkable films and to adhesive tapes having a heat shrinkable backing. More particularly, it relates to heat shrinkable pipewraps, e.g. for protecting from environmental forces and stresses the joints of pipeline such as those contemplated for inground implantation.
The concept of preparing heat shrinkable films is well known and the patent literature is replete with references thereto. In general, the prior procedures include the steps of crosslinking, e.g. chemically or by irradiation, and stretching to orient.
While not intended to be a complete survey of the rather extensive patent literature in what may be said to be a crowded art, the following patents are nevertheless considered to be relevant for a clearer understanding of the nature and objects of the present invention.
U.S. Pat. No. 3,022,543 relates to a shrinkable film for packaging foodstuffs, e.g. packaging meat by the cryovac process, the film being produced by the steps of: (1) stretching a polymer such as polyethylene where above room temperature in at least one direction; (2) cooling to at least room temperature; (3) irradiating, preferably with electrons at a specified dosage; (4) heating the irradiated film at a temperature where it will soften sufficiently to stretch; (5) bilaterally stretching to orient the polymer at this elevated temperature; and (6) cooling while under tension.
U.S. Pat. No. 3,144,398 relates to the preparation of irradiated polyethylene which can be readily changed from a stretched to a shrunken condition. The objectives are said to be accomplished by cold stretching the polyethylene at a temperature up to 65.degree. C., e.g. by cold calendering, and then irradiating at a specified dosage, preferably with electrons.
U.S. Pat. No. 3,144,399 relates to an irradiated, biaxially oriented polyethylene stretched at least 100% in each direction and below the break limit. The polyethylene is first irradiated at a specified dosage, then heated to or beyond its transparent point and stretched to orient. The stretched condition is maintained while cooling to room temperature.
U.S. Pat. No. 3,455,337 relates to a differentially irradiated crosslinked polymeric heat recoverable article, tubing being particularly disclosed, characterized by containing a sufficient crosslink density near one surface to be substantially infusible and containing a gradual decreasing density of crosslinking throughout its thickness, the opposed surface being substantially non-crosslinked. The article is formed by subjecting it to a dosage of irradiation insufficient to render the opposed surface infusible. Once the tubing has been differentially crosslinked, it is heated and then subjected to differential pressure between the inside and outside, the pressure being sufficient to cause the tube to expand in a controlled fashion.
U.S. Pat. No. 3,886,056 has for its objective to prepare from polyethylene having a high crystallinity a polyethylene having highly raised melting and softening temperatures, improved transparency and excellent dimensional stability at high temperatures. This objective is said to be accomplished by irradiating with a dosage of 0.2-16 Megarads to produce a crosslinked polyethylene having a gel content of at least one weight percent; extending the crosslinked polyethylene in at least one direction at a temperature of at least the anisotropic melting point of at least 150.degree. C.; and then cooling.
U.S. Pat. No. 3,949,110 discloses a method of making heat shrinkable tubing including the steps of irradiating the tube, heating to at least the softening temperature, partially inflating and then thermosetting by cooling.
U.S. Pat. No. 3,988,399 relates to heat recoverable wraparound sleeves for pipe joints, cables, wire splices and the like which posses the ability to curl in involute fashion upon the application of heat. As is disclosed, for example in Col. 6, one or both primary exterior faces can be coated with a suitable adhesive.
U.S. Pat. No. 4,348,438 discloses a process for preparing shrink wraps from a homopolymer of ethylene or a copolymer of ethylene with up to 20 weight per cent of one or more other monomers. As disclosed, the film is uniaxially cold-oriented at a temperature of at least 5.degree. C. below the film's melting point, irradiated with ionizing radiation and sealed along a seam running perpendicular to the direction of orientation. The orientation, which is preferably carried out before orientation, is at a dosage of 1-20 Megarads, about 3 to about 5 Megarads being stated to be preferred.
U.S. Pat. No. 4,469,742 relates to a multilayer cook-in shrink film comprising: (1) a specified sealing layer; (2) a shrink layer; (3) a specified adhesive layer; (4) a barrier layer; (5) another adhesive layer; and an abuse layer, the respective layers being melt bonded and irradiated to crosslink sufficient to resist delamination during use.
U.S. Pat. No. 4,517,234 relates to a flat length of heat recoverable material having integral latching means so that the material may be wrapped around a cable, pipe, etc., latched together and then shrunk.
U.S. Pat. No. 4,521,470 relates to a system for enclosing an object by installing consecutively or as a single article: (a) a heat-softenable adhesive; (b) a specified thermoplastic polymeric material; and (c) a heat-recoverable cover, and thereafter heating to cause recovery (shrink).
U.S. Pat. No. 4,590,020 teaches an oriented high density polyethylene film having maximum crosslinking at the surfaces and minimum crosslinking inwardly. The film is prepared by crosslinking opposed surfaces by irradiation with electron rays in such a manner that the degree of crosslinking decreases from the surfaces inwardly so that the outer layer portions have gel fractions between 20-70% and the middle layer portion has a gel fraction between 0-5%; and thereafter heating and stretching to orient and produce a film between 10-50 microns. As is stated in Col. 3, both sides of the film should be irradiated with the same dosage, the penetrating power of the electron rays being properly adjusted according to the thickness by changing the applied voltage or by using a shield.
As mentioned previously, the prior art is replete with references to shrink films and the aforementioned patents are not to be construed as constituting a full survey of the art. However, they are considered as illustrative for purposes of comprehending the nature and objects of the invention to be described in detail hereinafter.
In general, the present invention is directed to novel heat-shrinkable films and tapes, and to novel procedures for preparing same. More particularly, it is directed to novel protective wraps which are particularly useful for tubular metal objects such as pipes, e.g. pipelines intended for inground implantation, as well as for cables, wire splices and the like. The invention will accordingly be discussed in detail hereinafter by reference to pipewraps, a primary object of this invention.
It is well known in the art to provide an overwrap for pipelines in order to protect them from degradative stresses and forces after placement in the ground. An efficacious protective wrap of this description will in general comprise a rubber-based adhesive on the pipe surface and an outer tough corrosion and weather resistant film overlying the adhesive layer. Typically, this is accomplished by a protective adhesive wrap comprising a rubber-based adhesive carried on a polyolefinic film backing. Optionally, a primer coating may first be applied to the pipe surface in order to increase adhesion.
It would be most desirable to provide a so-called shrinkwrap of this description to insure complete adhesion of the tape to the pipe and thus avoid problems such a spiral void, as will be discussed in more detail hereinafter. However, prior systems for providing a polyethylene or other polyolefinic shrinkwrap for this purpose have proved inadequate for various reasons.
As is known, a polyolefinic film which has been oriented by stretching in the machine direction, will shrink when heated above its crystalline melting point due to the relaxation of the orientation. However, this concept is not in itself practical for preparing adhesive shrink wraps for pipes. The film backing for the adhesive will have no equilibrium or "rubbery" modulus above its melting point to generate the stresses necessary for adhesive flow. Accordingly, the tape will pull itself apart when it shrinks against a constraint, e.g. a pipe joint.
For example, a blend of high and low density polyethylene such as the backing on a 980 adhesive tape (trade designation of the Kendall Company, assignee of the instant invention), comprising a high density (HDPE):low density (LDPE) polyethylene blend, will begin to shrink when heated above the low density melting peak, but it will still maintain some strength if the shrink temperature is below the melting point of the HDPE. This could in theory be useful if the shrink temperature could be controlled to a narrow range of about 10.degree.-15.degree. C. between the two melting endotherms. However, this is not viable in contemplated commercial applications of heat to shrink the film.
It is also known that crosslinking will impart strength above the melting point to a polyolefinic film. For instance, a method of imparting strength to polyethyleme above its melting point(s) is to crosslink so that the film's 100% modulus at 150.degree. C. is between 25 and 100 psi. This has not previously been attractive for commercial production since the uniform crosslinking of the thick polyethylenes which would be utilized requires very high energy electron beams for irradiation which in turn would be both expensive and require considerable manufacturing floor space for radiation shielding. More importantly, the penetrating ionizing radiation passing through the film backing would destroy most, if not all of the contemplated rubber-based adhesives. In other words, an extraordinary R & D formulation effort would be required to provide a rubber-based adhesive suitable for the pipewrap which would not be severely damaged by the radiation.
On the other hand, applying the adhesive by calendering onto the surface of the film which has previously been crosslinked is also not feasible, as the adhesive would not adhere sufficiently.
A primary task of this invention, accordingly, is to provide a heat shrinkable polyolefin film which either carries a rubber-based adhesive layer, i.e. in the form of an adhesive tape, or which can be adhered securely over an adhesive layer applied in a separate step around the pipe or pipe joint.
Another object is to provide novel procedures for preparing heat shrinkable adhesive tapes, which procedures are simple, cost-effective and produce tapes possessing the adhesion and physical characteristics for protecting articles from the degradative environmental forces and stresses to which they are subjected in use.